Jig for firing of ceramic spark plug insulation

ABSTRACT

A jig for firing of stepped-diameter ceramic spark plug insulators has a bottom wall which is provided in its upwardly directed surface with a plurality of apertures each of which is adapted to accommodate a spark plug insulator during firing. Each of the apertures has a wider portion extending inwardly from the upper surface and emerging into a narrower portion which extends to the lower surface of the bottom wall. Side walls may or may not be provided extending upwardly from the upper surface.

I United States Patent 1191 [111 3,778,224

Duffner Dec. 11, 1973 [54] JIG FOR FIRING OF CERAMIC SPARK 2,499,634 3/1970 Reg 432/258 PLUG INSULATION FOREIGN PATENTS OR APPLICATIONS i751 invent: Baiiibeig, Geiiiiaiiy 889 726 9/1953 Germany 432/258 [73] Assignee: Robert Bosch G.m.b.H., Stuttgart,

Germany Primary Examiner-10hn J. Camby 22 Filed: Mal. 3 1972 At!0rneyMichael S. Striker [21] Appl. No.: 231,606 [57] ABSTRACT [30] Foreign Ap li ti P i it D t A jig for firing of stepped-diameter ceramic spark plug Mar. 16 1971 Germany P 21 12 507.5 iiisiiiaiois has a Waii which is Piovided in its upwardly directed surface with a plurality of apertures [52] U.S. Cl. 432/258 each of which is adapted to accommodate a Spark 51 1m. (:1. r271) 21/00 P insulator during firing- Each the apertures has 58 Field of Search 432/258- 264/57 wider extending inwmiiy from "PPer 2 surface and emerging into a narrower portion which extends to the lower surface of the bottom wall. Side [56] Reerences Cited yvalls gray or may nfot be provlded extendmg upwardly UNITED STATES PATENTS mm 6 upper 1,513,617 10/1924 Litt 264/57 6 Claims, 3 Drawing Figures i ii I i 1'1! v- 1+ i I I v i i i .1% l L i 1 A VA MG FOR FIRING OF CERAMIC SPARK PLUG INSULATION BACKGROUND OF THE INVENTION The present invention relates to a jig for firing ceramic spark plug insulators, and more particularly to a novel jig for firing of such insulators in a high temperature oven.

Spark plug insulators are conventionally made of a suitable ceramic material, or a mixture of such materials, and are shaped to the desired configuration. However, when they are so shaped from such material, the insulators are soft and can be given the necessary high mechanical strength and electrical insulation characteristics only by being fired in -a firing oven, usually at temperatures of between substantially 1,500 and 1,650C. When these blanks, that is the spark plug insulators which have been formed from ceramic material but have not yet been fired, are to be introduced into the firing oven, they are conventionally inserted into a jig provided for this purpose, usually called a burner capsule. Such jigs are as a rule configurated as containers of rectangular or quadratic outline and are themselves bodies consisting of fire-proof ceramic ma-' terial.

The jigs must of course have sufficient mechanical strength, in order to be capable of supporting and retaining the ceramic spark plug insulators even at the highest firing temperatures which are to be encountered. To assure this it was heretofore thought that particularly the bottom wall of the jig must be very thick. This, of course, results in a significant reduction of the useable proportion of the interior space of the firing oven, especially in view of the fact that it is conventional to stack six of these jigs above one another so that in height six times the thickness of the bottom wall of the respective jig is lost as useable space. These jigs frequently have a bottom wall thickness of 22 mm, meaning that if six of them are stacked above one another, a height of 132 mm in the chamber of the firing oven is lost as useable space, that is as space where ceramic spark plug insulators could potentially be located for firing purposes.

Evidently, this is a disadvantage attendant to the prior art. An additional disadvantage is to be found in the fact that these known jigs cannot be produced according to precise dimensions. They shrink by substantially 1-2 percent during their first firing, that is when they themselves are tired to impart to them the necessary mechanical strength, and during further firings when they support ceramic spark plug insulators they may undergo other dimensional changes, for instance they may also increase in size. As a result of all this it has been found that it is to all intents and purposes impossible to produce a jig in which the spark plug insulators will always be in contact with one another and with the walls bounding the jig, without any freedom of play. Another reason why this is impossible resides in the fact that the soft un-fired spark plug insulator blanks also differ from one another in their diameters.

For the above dimensional reasons it has heretofore been necessary to tightly pack or position spark plug insulatorsto be fired in the jigs of the prior art, carrying this out by hand and filling any remaining gaps between adjacent insulators with ceramic filler elements and wedges, a procedure which is necessary to assure that during the transportation of the jig into the firing oven the not-yet-fired spark plug insulators cannot move relative to one another and thus become damaged.

From what has been set forth above, it is clear that a significant disadvantage of the prior art resides in the fact that the utilization of the prio:r-art jigs is time consuming, thus increasing the expenses involved in producing the spark plug insulators and, ultimately, producing the spark plug itself.

SUMMARY OF THE INVENTION Accordingly, it is a general object of the present invention to overcome the disadvantages of the prior art.

More particularly, it is an object of the present invention to produce an improved jig for firing of ceramic spark plug insulators, which is not possessed of the aforementioned disadvantages.

A concomitant object of the invention is to provide such an improved jig of the type here under discussion, in which the difficulty and time consumed in handling the jig and the insulators are substantially reduced.

Still a further object of the invention is to provide such an improved jig in which it is possible to insert the spark plug insulators into the jig mechanically and automatically, rather than manually.

Still another object of the invention is to provide such a jig which reliably prevents damage of the inserted spark plug insulators.

In pursuance of these objects and of others which will become apparent hereafter, one feature of the invention resides in a jig for firing of stepped-diameter ceramic insulators in a high-temperature oven, which jig briefly stated comprises a body of fireproof material, said body having a bottom wall having two opposite major surfaces one of which faces upwardly and the other of which faces downwardly when the jig is in use. The bottom wall is provided with a plurality of apertures each adapted to receive and retain a spark plug insulator. Each of these apertures has a wider portion extending from the one towards the other surface and a narrower portion extending from the wider portion to the other surface. The wider and narrower portions are adapted to respectively receive a large-diameter and a small-diameter section of a stepped-diameter spark plug insulator.

According to a further concept of the invention, the novel jig is not provided with four side walls as in the prior art, but instead with at most two side walls on which additional jigs may be stacked.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a partially sectioned perspective view illustrating a jig according to the prior art;

FIG. 2 is a view similar to FIG. 1., but illustrating a jig according to the present invention;

FIG. 3 is a sectional elevation showing the novel jig with spark plug insulators inserted and illustrating means for ejecting the spark plug insulators after firing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Discussing the drawing in detail, and firstly the priorart embodiment illustrated in FIG. 1, it will be seen that this jig is in the form of an essentially cup-shaped element of quadratic or rectangular configuration. This prior-art jig has four side walls and a bottom wall 11, and is usually of ceramic material. Of course, in the illustrated embodiment one of the side walls is not visible but it will be understood that the jig does have four of the side walls 10. The thickness of the bottom wall 11 is conventionally between 20 and 25 mm.

These jigs are filled with spark plug insulators, or more particularly with soft blanks for such insulators, which are to be fired in order to be hardened. These insulators, of which one is illustrated, are of stepped diameter, having a wider section 13 and a narrower section 12. They are so inserted into the jig that they stand therein in upright condition and that adjacent ones of the insulators contact one another with their wider section 13. However, as already pointed out earlier, it is inevitable that spaces will remain between the insulators and these spaces between the insulators and between the insulators and the side walls 10 must be filled by ceramic inserts or wedges (not illustrated) in order to prevent relative movement of the insulators and possible damage before they can become fired.

It will be clear that this approach in the prior art results in a rather time-consuming loading operation, inasmuch as the insulators 12, 13 must be inserted manually, and in view of the fact that after the jig is filled with the insulators, the ceramic inserts or wedges must also be placed manually.

By contrast the embodiment in FIGS. 2 and 3, shows a jig according to the present invention which avoids the disadvantages of the prior art.

The jig in FIGS. 2 and 3 is also of ceramic material, particularly a ceramic material on the basis of alumina/mullite, the porosity of which is between 20 and 30 percent, having a bottom wall 14 which is provided with a plurality of apertures each having a portion 15 which is wider and which extends downwardly from the normally upper surface of the bottom wall 14, a portion 17 which extends from the region of the portion 15 towards the normally lower surface of the bottom wall 14 and which is narrower than the portion 15 and an intermediate portion 16. The portions 15 and 17 are concentric with one another, and the portions 15 are, of course, adapted to accommodate the sections 12 (see FIG. 1) of the insulator blanks.

The spacing between the centers of these apertures, for instance of the sections 15, is so selected that it is only slightly greater than the diameter of the insulator sections 13, in order to assure maximum utilization of the surface area of the bottom wall 14. This means that prior to firing, the sections 13 of the various insulator blanks will either contact one another, that is the sections 13 of adjacent blanks will contact one another, or there will be very small play between the blanks when an insulator blank is inserted into each of the apertures of the jig.

The jig according to the present invention is filled with insulator blanks, not manually, but automatically, for which purpose the jig is placed onto a suitable conveyor which moves along and beneath a device which sequentially inserts blanks into the variousapertures of the jig. This arrangement for moving the jig and inserting the blanks is not a part of the present invention, except that it is pointed out that the utilization of such a device is possible for the first time due to the particular configuration of the novel jig.

Of course, the novel jig will undergo dimensional changes during repeated firing, as does the jig known from the prior art. As a result of this the spacing between the apertures in the jig will also change over a period of time and the differences between the always identical spacing between adjacent inserting stations of the automatic inserting device and the changes in the dimensions between the adjacent apertures in the jig will be cumulative over a period of time so that when dimensional changes in the jig have occurred, in subsequent filling operations the apertures of the jig will no longer be properly aligned with the filling stations of the automatic filling device during the entire sequence of filling operations. This means that proper insertion of an insulator blank at each filling station into a respective aperture of the jig would not be assured any more. This, however, can be avoided by providing a non-illustrated guide pin which extends from below into the aperture portions 17 and shifts the jig with respect to the device until the aperture portions 17 again register with the respective filling station. This also is not a part of the present invention and is mentioned only for completeness of description.

It should not be assumed, however, that the aperture portions 17 are provided only in order to permit the insertion of the aforementioned pin. Rather, they have other purposes also. For instance, during the firing process the blanks 18 (illustrated in FIG. 3) will contract in their diameter and length by approximately 20 percent, so that they have rather large play in the apertures. In order to assure that no damage occurs to the converted blanks, that is to the fired insulators, which still are susceptible to such damage even after firing, pins 19 shown in FIG. 3 are inserted after firing from below into the aperture portions 17, lifting the fired insulators upwardly until they can be gripped by suitable gripping devices at the junction between their sections 12 and 13, and can be removed automatically to be deposited on repositories provided for this purpose.

It will be appreciated that the novel jig thus permits not only the automatic loading but also the automatic unloading of the jig, thus increasing substantially the speed of loading and unloading and decreasing substantially the expenses involved in these operations.

Moreover, the provision of the apertures in the bottom wall 14 of the novel jig increases the capacity of the jig for handling the insulators. Unlike the conventional jigs, such as shown in FIG. 1, the novel jig according to the present invention need have a bottom wall 14 which has a thickness of only about 7 mm. as opposed to the 22 mm of the prior art. This means that for each jig a thickness of 15 mm. is saved and when six jigs are stacked above one another in the firing cham ber of a high-temperature oven, a total of mm. can be saved. This dimension corresponds approximately to the height of one of the jigs so that it is now possible to stack seven jigs in the space which previously was required 'for only six jigs, without having to change the dimensions of the oven chamber. This means an increase of 16.7 percent in the number of spark plug insulators which can be fired at one time.

Furthermore, due to the fact that the jig according to the present invention need have only two side walls 20 (see FIG. 2) instead of four side walls as in the priorart construction shown in FIG. 1, a further increase in the ability of the jig to hold spark plug insulators is achieved, amounting to 7 percent because the space previously required for the two omitted side walls can now be used for apertures in which spark plug insulators can be accommodated. The overall increase in the number of spark plug insulators which can be fired at one time, achieved by the two measures mentioned above, thus amounts to 24 percent over the prior art. The support of upper jigs, that is those stacked on a lower jig, is provided by the two side walls 20, or rather by their upper edges. In fact, it is possible to completely eliminate the side walls and to use loosely inserted supports such as pins or the like located for instance at the four corners of the bottom wall 14; such pins can for instance be inserted into four corner apertures of the bottom wall 14. However, this provides for lesser stability of a stack of jigs and is a solution which is not currently preferred, although it should be kept in mind that it does present a possible alternative to the provision of the side walls 20.

It will be understood that each of the elements described above or two or more together, may also find a useful application in other types of constructions differing from the types described above.

While the invention has been illustrated and described as embodied in a jig for firing of steppeddiarneter ceramic spark plug insulators, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge readily adapt it for various applications without omitting features that from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

1. A jig for firing of ceramic spark plug insulators in a high-temperature oven, comprising a body of fireproof material, said body including a bottom wall having two opposite major surfaces one of which faces upwardly and the other of which faces down-wardly when said jig is in use; and a plurality of apertures provided in said bottom wall, each of said apertures having a wider portion extending from said one towards said other surface, and a narrower portion extending from said wider portion to said other surface so that a shoulder is defined at the juncture of the respective wider and narrower portions, each of said wider portions being adapted to receive a section of a spark plug insulator and to retain the latter resting upon the respective shoulder.

2. A jig as defined in claim 1, said body being of ceramic material.

3. A jig as defined in claim 1, said body further having side walls extending upwardly beyond said one surface and adapted to support a similar jig on their respective upper edges.

4. A jig as defined in claim 1, wherein the thickness of said bottom wall intermediate said surfaces is substantially 7 mm.

5. A jig as defined in claim 1, said body further having only two side walls extending upwardly beyond said one surface and adapted to support a similar jig on their respective upper edges.

6. A jig as defined in claim 1, wherein the wider and narrower portions of the respective apertures are arranged concentrically. 

1. A jig for firing of ceramic spark plug insulators in a hightemperature oven, comprising a body of fireproof material, said body including a bottom wall having two opposite major surfaces one of which faces upwardly and the other of which faces downwardly when said jig is in use; and a plurality of apertures provided in said bottom wall, each of said apertures having a wider portion extending from said one towards said other surface, and a narrower portion extending from said wider portion to said other surface so that a shoulder is defined at the juncture of the respective wider and narrower portions, each of said wider portions being adapted to receive a section of a spark plug insulator and to retain the latter resting upon the respective shoulder.
 2. A jig as defined in claim 1, said body being of ceramic material.
 3. A jig as defined in claim 1, said body further having side walls extending upwardly beyond said one surface and adapted to support a similar jig on their respective upper edges.
 4. A jig as defined in claim 1, wherein the thickness of said bottom wall intermediate said surfaces is substantially 7 mm.
 5. A jig as defined in claim 1, said body further having only two side walls extending upwardly beyond said one surface and adapted to support a similar jig on their respective upper edges.
 6. A jig as defined in claim 1, wherein the wider and narrower portions of the respective apertures are arranged concentrically. 